An Arrayed Waveguide Grating (AWG) is a device commonly used as a frequency or wavelength optical demultiplexer and can be considered as a spatially dispersive lens. For example, an image received at an input of the AWG will be projected onto an output plane similar to a conventional lens, but the position of the output image on the output plane is wavelength dependent. Accordingly, the position of the output image may change as the wavelength of the input image changes, or as the input position of the input image changes.
FIG. 10 illustrates components of a conventional AWG 1100. As shown in FIG. 10, an input image 1114 may be projected from an input waveguide 1102 into an input free-space propagation region 1104, and the image may expand, or diffract, within the input free-space propagation region 1104. A waveguide array 1106 may be disposed at an opposite end of the input free-space propagation region 1104 to collect the expanded image. Waveguide array 1106 comprises a series of quasi-parallel waveguides where the length of each waveguide increases by a constant and specified amount from an adjacent waveguide. That is, starting from the innermost shortest waveguide, each subsequent waveguide increases in length by a specified amount.
The collected image is received by the waveguide array 1106 and projected into an output free-space propagation region 1108. The image output from waveguide array 1106 propagates through the output free-space propagation region 1108 onto the output image plane 1110 containing output waveguide 1112. As generally understood, due to phase curvature and phase tilt of light propagating through waveguide array 1106, the image may be refocused on to an output image plane 1110. The output image 1116 is initially received at the right side 1110a in FIG. 10 of the output image plane 1110 and scans from the right side 1110a to the left side 1110b as indicated by arrow 1118, as the wavelength changes. Put another way, the position of the output image 1116 on the output image plane 1110 may change as a function of wavelength, and the change in the location of the image 1116 across the output image plane 1110 of the AWG 1100 is generally referred to as the scanning property of the AWG.
The output waveguide 1112 collects the image as it scans across the output image plane 1110. Typically, the input waveguide 1102 and the output waveguide 1112 have the same dimensions such that, in an ideal AWG, the output image will substantially match a mode profile of the output waveguide. Accordingly, when the output image 1116 is centered on an output waveguide, the transmission response from input to output is nearly 100% (i.e. unity). Because the position of the output image 1116 changes with frequency, multiple output waveguides 1112 may be provided to collect light at different frequencies, such that, in one example, AWG 1100 may function as an optical demultiplexer. Output waveguide 1112 collects the maximum amount of energy at a predetermined frequency or wavelength, whereby the output image 1116 is centered on output waveguide 1112. The amount of energy collected by an output waveguide 1112 is reduced as the frequency of the output image 1116 varies from the center frequency of the output waveguide 1112. That is, a passband associated with output waveguide 1112 is relatively narrow such that optical signals at wavelengths that are shifted from a peak transmission wavelength or center wavelength of the passband may incur substantial loss.
Optical signals supplied to the AWG may have varying wavelengths or wavelengths that are offset from the predetermined wavelength at which the output image 1116 would be centered on output waveguide 1112 due to system tolerances and non-idealities. Accordingly, such signal my incur loss during propagation through the AWG. Moreover, the passband or full-width half maximum (FWHM) associated with output waveguide 1112 is typically 40%-50% of a wavelength spacing separating the optical signal wavelengths. Thus, in systems in which the optical signal wavelengths are narrowly spaced, minor deviations in optical signal wavelengths can result in loss, as the optical signals pass through the AWG.
Accordingly, there is a need for an AWG that can multiplex/demultiplex optical signals with reduced loss, even when the optical signal wavelengths are offset from a center wavelength of an AWG passband.